Patent EA-001917-B1 (EA)
Title: Removal of silica from bauxite
Inventor: HOLLITT MICHAEL JOHN
IPC: C01F7/062
Filed: 1997-11-20
Abstract: 1. A process for the removal of reactive silica from feedstocks to the digestion step of the Bayer process for the manufacture of alumina comprising the following steps: (a) contacting the Bayer process feedstock with a caustic liquor under the following process conditions which result in dissolving and stabilising at least 50% of the silica into solution at a level of at least 3 gpL and without significant precipitation from solution of the dissolved silica: (i) process exit liquor composition: - caustic strength (as Na2CO3) greater than 250 gpL; - alumina concentration (as A12O3) greater than 125 gpL; (ii) temperature: 60 degree C - 125 degree C; (iii) contact time: greater than 20 minutes; and (b) separating the silica bearing liquor from the solid residue of silica dissolution step (a) under conditions which do not promote significant precipitation of the silica, treating the silica bearing liquor from separation step (b) under conditions which encourage precipitation of aluminosilicates to form a solid aluminosilicate product which is free of the majority of all other components of the feedstock. 2. The process defined in claim 1 wherein the digestion of alumina from the feedstock is less than 20% under the process conditions of silica dissolution step (a). 3. The process defined in claim 1 wherein the precipitation conditions comprise one or more than one of the addition of seed, by heating, by dilution and the addition of additives. 4. The process defined in any one of the preceding claims wherein the process conditions of silica dissolution step (a) result in dissolving and stabilising at least 80% of the silica into solution. 5. The process defined in claim 4 wherein the process conditions of silica dissolution step (a) result in dissolving and stabilising at least 90% of the silica into solution. 6. The process defined in any one of the preceding claims wherein the level of dissolved and stabilised silica in dissolution step (a) is 6-15 gpL. 7. The process defined in any one of the preceding claims wherein the caustic strength of the caustic liquor (as Na2CO3) in silica dissolution step (a) is greater than 300 gpL. 8. The process defined in claim 8 wherein the caustic strength of the caustic liquor (as Na2CO3) is greater than 350 gpL. 9. The process defined in any one of the preceding claims wherein in the alumina concentration of the caustic liquor (as Al2O3)in silica dissolution step (a) is greater than 150 gpL. 10 The process defined in claim 9 wherein the alumina concentration of the caustic liquor (as Al2O3) in silica dissolution step (a) is greater than 175 gpL. 11. The process defined in any one of the preceding claims wherein the temperature of the silica dissolution step is 80 degree C - 105 degree C. 12. The process defined in any one of the preceding claims wherein the contact time of the silica dissolution step (a) is at least 1 hour. 13. The process defined in claim 12 wherein the contact time is 2-4 hours. 14. The process defined in any one of the preceding claims wherein the dissolved silica content of the caustic liquor supplied to the silica dissolution step is less than 2 gpL. 15. The process defined in any one of the preceding claims wherein the solids loading of the feedstock/caustic liquor mixture in silica dissolution step (a) is 50-700 gpL. 16. An aluminosilicate product made by the Bayer process defined in any one of the preceding claims. 17. A process as defined in claims 1 to 15 wherein the feedstock comprises greater than 5% boehmite or diaspore.